Compounding organic or inorganic materials into adhesives, exterior coatings, hard coats, antireflection coatings, and the like has been studied to improve scratch resistance and strength after cure, adhesion to substrates, and the like.
In mixing an organic material and an inorganic material, it is necessary to prevent undesired agglomeration of the inorganic material. One of the methods generally followed is to disperse an inorganic material in a solvent compatible with an organic material and mix the dispersion and the organic material to prepare a coating composition for forming a coating film. In order to secure stable performance, it is important that the inorganic material be stably dispersed in the solvent. In other words, it is important to control the hydrophilicity/hydrophobicity or steric hindrance of the surface of the inorganic material. Surface treatment of fine inorganic oxide particles with an alkoxysilane is known. For example, Ganryo Bunsan Gijutu Hyomenshori to Bensanzai no Tukaikata oyobi Bunsansei Hyoka, Technical Information Institute Co., Ltd., 1999 describes a technique of dispersing inorganic particles in an organic solvent with the aid of a silane coupling agent. The technique is insufficient in terms of stability of a resultant dispersion, nevertheless.
In combining an organic material particularly of polymerization cure system and inorganic particles, use of an alkoxysilane having a polymerizable group and/or a hydrolysis-condensation product thereof has been attracting attention. For instance, JP-A-9-169847 proposes a combined use of a specific polyalkoxypolysiloxane and a polymerizable silane coupling agent. However, the reaction between the polyalkoxypolysiloxane and the polymerizable silane coupling agent hardly proceeds sufficiently for achieving a high ratio of introducing the polymerizable group so that the resulting cured product is not satisfactory in scratch resistance or strength. JP-A-9-40909 proposes use of a partial co-hydrolysis-condensation product of an alkoxysilane having an organic functional group and a tetraalkoxysilane, but the coating composition containing the same has insufficient storage stability, still leaving room for further improvement.
An optical film, particularly an antireflection film is generally disposed in front of displays, such as cathode ray tube displays (CRTs), plasma display panels (PDPs), electroluminescence displays (ELDs), and liquid crystal displays (LCDs), whereby to prevent contrast reduction or reduction of visibility due to reflection of ambient light by making use of optical interference.
Antireflection films for that application are manufactured by forming a low reflective layer with a proper thickness on a substrate and, if desired, other layers including a high reflective layer, a middle reflective layer, and a hard coat layer, between the low reflective layer and the substrate. To achieve a low reflectance, it is desirable that the low reflective layer be made of a material having as low a refractive index as possible. Disposed as an outermost layer of displays, the antireflection film is required to have high scratch resistance. To secure high scratch resistance with as small a thickness as about 100 nm, the antireflection film should exhibit strength per se and adhesion to an underlying layer.
Means for reducing a refractive index of a material include introducing fluorine and decreasing the density (by making voids), but both approaches are accompanied by reductions in film strength (scratch resistance) and adhesion. It has therefore been a difficult problem to satisfy both the requirements for low refractive index and high scratch resistance.
JP-A-11-189621, JP-A-11-228631, and JP-A-2000-313709 disclose techniques for providing a film with a reduced frictional coefficient and improved scratch resistance by introducing a polysiloxane structure into a fluoropolymer. The technique is effective to some extent in improving scratch resistance but is still insufficient for improving scratch resistance of a coating film that essentially lacks film strength and interfacial adhesion.
JP-A-2003-222704 teaches that addition of a silane coupling agent to a low refractive layer material containing a fluoropolymer provides a coating film with markedly improved scratch resistance. However, because a silane coupling agent having a low boiling point vaporizes while being applied and dried, it must be added in an excess corresponding to the evaporation loss, which makes it difficult to obtain stable performance.